Semiconductor lasers represent one of the most important class of lasers in use today because they can be used in a wide variety of applications including displays, solid-state lighting, sensing, printing, and telecommunications just to name a few. The two types of semiconductor lasers primarily in use are edge-emitting lasers and surface-emitting lasers. Edge-emitting lasers generate light traveling in a direction substantially parallel to the light-emitting layer. On the other hand, surface-emitting lasers generate light traveling normal to the light-emitting layer. Surface-emitting layers have a number of advantages over typical edge-emitting lasers: they emit light more efficiently and can be arranged to form two-dimensional, light-emitting arrays.
Surface-emitting lasers configured with the light-emitting layer sandwiched between two reflectors are referred to as vertical-cavity surface-emitting lasers (“VCSELs”). The reflectors are typically distributed Bragg reflectors (“DBRs”) that ideally form a reflective cavity with greater than 99% reflectivity for optical feedback. DBRs are composed of multiple alternating layers, each layer composed of a dielectric or semiconductor material with periodic refractive index variation. Two adjacent layers within a DBR have different refractive indices and are referred to as “DBR pairs.” DBR reflectivity and bandwidth depend on the refractive-index contrast of constituent materials of each layer and on the thickness of each layer. The materials used to form DBR pairs typically have similar compositions and, therefore, have relatively small refractive-index differences. Thus, in order to achieve a cavity reflectivity of greater than 99%, and provide a narrow mirror bandwidth, DBRs are configured with anywhere from about 15 to about 40 or more DBR pairs. However, fabricating DBRs with greater than 99% reflectivity has proven to be difficult, especially for VCSELs designed to emit light with wavelengths in the blue-green and long-infrared portions of the electromagnetic spectrum.
Physicists and engineers continue to seek improvements in VCSEL design, operation, and efficiency.